The present invention generally relates to image reproduction methods and systems, and more particularly to a natural image reproduction method and system applicable to displaying and printing of a still image.
Unlike character information and graphic information, the information quantity of the natural or multi-level image is considerably large. For this reason, the natural image information is normally stored in a coded and compressed form.
When storing the natural image information in the compressed form, there is a demand to shorten the processing time when displaying or printing the natural image information which is stored in the compressed form.
Conventionally, the natural image information is stored in the coded and compressed form using a coding system such as the adaptive discrete cosine transform (ADCT) coding and the generalized block truncation coding (GBTC). In such cases, the natural image information is compressed with a compression rate on the average of 1/10.
However, the information quantity is still large even after the coding described above. For example, in the case of the natural image information related to 600.times.400 pixels, the information quantity after the coding is approximately 80 kbits. On the other hand, when the natural image is displayed by the simple operation of successively restoring the coded data, it takes approximately 10 seconds to display the natural image of the 600.times.400 pixels. For this reason, the coding is first carried out with respect to the rough image in its entirety, and the coding is thereafter carried out with respect to the fine image.
When displaying the natural image which is coded in two stages as described above, the rough image is first displayed so that the general image information can be recognized, and the fine image is thereafter displayed gradually with time. Such a method of reproducing the stored image information is called a progressive display. The progressive display itself is known from Trikepse SS 9, pp.193-196, for example.
FIGS. 1A, 1B and 1C respectively show photographs of a natural image which is displayed by the progressive display. The display is made in a sequence starting from the image shown in FIG. 1A to the final image shown in FIG. 1C, where the image shown in FIG. 1B is finer than that shown in FIG. 1A and the final image shown in FIG. 1C is finer than that shown in FIG. 1B.
The progressive display method can roughly be divided into three display steps.
FIGS. 2A, 2B and 2C respectively show a natural image which is represented by the three display steps of the progressive display method. The image becomes finer in the sequence of FIGS. 2A, 2B and 2C. FIG. 2A shows a retrieval image which is used to obtain a general idea of the image, and it takes approximately 3 seconds to display such a rough image. FIG. 2B shows a displaying image which is used for display on a display device such as a cathode ray tube (CRT), and it takes approximately 10 seconds to display such a soft copy image. FIG. 2C shows a printing image which is to be printed, and it takes approximately 20 seconds to display such a hard copy image. Although the display of the hard copy image can be obtained in less than one minute, if no display were made during this time, the user may think that something is wrong. In addition, the waiting time of less than one minute may seem very long to the user when no display is made during this time. Hence, it is very helpful to display the retrieval image first so that the user is aware of the process which is being made.
FIG. 3 shows a conventional image data compression process. A step S1 generates the retrieval image data for the entire image by coding the natural image data. A step S2 generates the soft copy image data for the entire image by coding the difference between the retrieval image and the soft copy image. A step S3 generates the hard copy image data for the entire image by coding the difference between the soft copy image and the hard copy image.
For example, the compressed image data are stored in a memory as shown in FIG. 4. The retrieval image data, the soft copy image data and the hard copy image data are respectively stored in files RI, SI and HI. These image data are read out and decoded when displaying and/or printing the hard copy image.
When the information quantity of the retrieval image data is taken as being "1", the information quantity of the soft copy image data is "2.5" and the information quantity of the hard copy image data is "3.75".
The retrieval image is restored by decoding the retrieval image data. The soft copy image is restored by decoding the soft copy image data and adding the restored data to the restored data related to the retrieval image. The hard copy image is restored by decoding the hard copy image data and adding the restored data to the restored data related to the soft copy image.
The soft copy image and the hard copy image cannot be distinguished from each other when displayed on the display device due to the relatively poor resolution of the display device. However, when printing the natural image, the soft copy image cannot be used and it is essential to use the hard copy image because of the high resolution required in the printer. As a result, it is necessary to wait for the hard copy image to be restored before starting the printer, and there is a problem in that the printing time as a whole becomes relatively long due to the waiting time.